The present invention relates to static dissipative polyurethane foam suitable for making rollers that electrostatically control the object to be contacted, such as toner transfer rollers, charging rollers, development rollers and cleaning rollers used in a printer for electrophotography and electrostatic recording systems. Such foams also find use in other applications, such as cosmetic pads, clean room wipes and electrical component packaging.
Rollers are used for example in the development mechanism and the cleaning mechanism of electrophotographic systems. Such rollers should be stable over different environmental conditions, such that conductivity or static dissipative characteristics do not fluctuate substantially. For example, surface resistivity should remain within one order of magnitude, i.e., between 1×108 ohms/square and 1×109 ohms/square when the ambient conditions are between 5° C. at 30% relative humidity and 30° C. at 85% relative humidity.
Surface resistivity is the ratio of DC voltage drop per unit length to the surface current per unit width. In effect the surface resistivity is the resistance between two opposite sides of a square and is independent of the size of the square or its dimensional units. Generally, conductive materials have surface resistivity values of less than 1×103 ohms/square, static dissipative materials have surface resistivity values less than 1×1012 ohms/square and insulative materials have surface resistivity values of greater than 1×1012 ohms/square. Untreated polyurethane foams are most commonly insulative materials with surface resistivities greater than 1×1014 ohms/square, and even greater than 1×1017 ohms/square. Such surface resistivity classifications are found in ASTM Test method D-257. diisocyanate, and a quaternary ammonium salt is added to the foaming mix to impart electroconductivity. The resulting foams have volume electrical resistivity from 1×106 ohm-cm to 1×1011 ohm-cm.
U.S. Pat. No. 5,677,357 suggests adding an anti-statically-effective amount of an inorganic hexahalogenated compound to a polyurethane composition. U.S. Pat. No. 5,955,526 teaches adding a hexahalogenated phosphate compound to a polyurethane composition. The resulting polyurethane foams in these patents are reported to have surface resistivities in the static dissipative range, i.e., below 1×1012 ohms/square.
For rollers formed from polyurethane foams, in addition to surface resistivity, other important parameters are foam density, hardness and cell size. Commonly, rollers are fabricated by foaming the foam within an enclosed mold cavity. The contours of the mold cavity closely approximate the desired shape of the finished roller. With such molding, the foam can develop with higher densities, e.g., greater than 10 pounds per cubic foot (160 kg/m3).
Moreover, foams with finer cell sizes, e.g., greater than 100 pores per inch, exhibit better electroconductivity. Such foams with finer cell size are also desired for other applications, such as clean room wipes and cosmetic applicators.
Fabricating rollers and other shaped polyurethane foam parts by molding generally is more expensive than fabricating such rollers and parts from free rise foams. “Molded” herein means reacting the foam-forming ingredients while they are confined within an enclosed mold cavity. The molded parts require preparation of molds tailored to the specific dimensions of the roller or part structure. A new mold must be built to create a new shaped foam roller or part. The foam properties can vary through the volume and across the surface of the molded foam part.
Volume resistivity is the ratio of the DC voltage per unit thickness to the amount of current per unit area passing through a material. Volume resistivity is generally given in ohm-cm.
Electroconductive or static dissipative polyurethane foams are known and have been produced by various methods, including:                (a) blending electroconductive carbon (or other metallic) particles with a mixture of polyol, isocyanate(s), catalyst(s), water and foaming agent(s) when foaming the polyurethane foam to form a conductive or static dissipative foam in situ;        (b) impregnating a polyurethane foam with a carbon paint or liquid solution containing electroconductive carbon (or other metallic) particles; and        (c) adding ionic anti-static agents to the polyurethane foam-forming mix to form a conductive or static dissipative foam in situ.        
U.S. Pat. No. 5,656,344 discloses an electroconductive polyurethane foam for the surface of a printer roller. The foam incorporates electroconductive metal powders, preferably carbon or graphite, and ionic conductive material such as metal salts or other anti-static agents. The resulting foam has a volume resistivity in the range of 1×106 to 1×1011 ohm-cm. Such volume resistivity is stated to remain stable such that it does not fluctuate more than one order of magnitude for a range of ambient conditions from about 5° C. at 30% relative humidity to about 35° C. at 85% relative humidity. The foams are molded to form rollers.
U.S. Pat. No. 5,933,693 discloses other molded electroconductive polyurethane foams and elastomers that may be used to form rollers for electrophotographic or electrostatic recording processes. The foam is formed with specific modified or hydrogenated diphenyl methane Molded foam parts commonly have a densification or skin formed at the outer surface that was in contact with the mold.
In contrast, free rise foam is formed into a large-sized bun with generally consistent properties through the volume of the bun. “Free rise” herein means unhampered expansion of the foam in an open container or on a moving conveyor with no top. The container or conveyor may be within a pressure-controlled environment, but there is no surface equivalent to a mold surface limiting foam expansion. After curing, the free rise foam can be shaped or fabricated on standard equipment by machining, cutting, peeling, embossing, or other mechanical or mechanical-thermal means to form rollers or other shaped parts.
Lower cost static dissipative polyurethane foams with sufficiently high density, desired hardness, sufficiently fine cell size and sufficiently stable surface resistivities are still sought for rollers and other applications.